GIT-SUBTREE

 

NAME

git-subtree – Merge subtrees together and split repository into subtrees  

SYNOPSIS

git subtree add   -P <prefix> <commit>
git subtree add   -P <prefix> <repository> <ref>
git subtree pull  -P <prefix> <repository> <ref>
git subtree push  -P <prefix> <repository> <ref>
git subtree merge -P <prefix> <commit>
git subtree split -P <prefix> [OPTIONS] [<commit>]

 

DESCRIPTION

Subtrees allow subprojects to be included within a subdirectory of the main project, optionally including the subproject’s entire history.

For example, you could include the source code for a library as a subdirectory of your application.

Subtrees are not to be confused with submodules, which are meant for the same task. Unlike submodules, subtrees do not need any special constructions (like .gitmodule files or gitlinks) be present in your repository, and do not force end-users of your repository to do anything special or to understand how subtrees work. A subtree is just a subdirectory that can be committed to, branched, and merged along with your project in any way you want.

They are also not to be confused with using the subtree merge strategy. The main difference is that, besides merging the other project as a subdirectory, you can also extract the entire history of a subdirectory from your project and make it into a standalone project. Unlike the subtree merge strategy you can alternate back and forth between these two operations. If the standalone library gets updated, you can automatically merge the changes into your project; if you update the library inside your project, you can "split" the changes back out again and merge them back into the library project.

For example, if a library you made for one application ends up being useful elsewhere, you can extract its entire history and publish that as its own git repository, without accidentally intermingling the history of your application project.

Tip

In order to keep your commit messages clean, we recommend that people split their commits between the subtrees and the main project as much as possible. That is, if you make a change that affects both the library and the main application, commit it in two pieces. That way, when you split the library commits out later, their descriptions will still make sense. But if this isn’t important to you, it’s not necessary. git subtree will simply leave out the non-library-related parts of the commit when it splits it out into the subproject later.

 

COMMANDS

add

Create the <prefix> subtree by importing its contents from the given <commit> or <repository> and remote <ref>. A new commit is created automatically, joining the imported project’s history with your own. With –squash, imports only a single commit from the subproject, rather than its entire history.

merge

Merge recent changes up to <commit> into the <prefix> subtree. As with normal git merge, this doesn’t remove your own local changes; it just merges those changes into the latest <commit>. With –squash, creates only one commit that contains all the changes, rather than merging in the entire history.

If you use ‘–squash’, the merge direction doesn’t
always have to be forward; you can use this command to
go back in time from v2.5 to v2.4, for example.  If your
merge introduces a conflict, you can resolve it in the
usual ways.

pull

Exactly like merge, but parallels git pull in that it fetches the given ref from the specified remote repository.

push

Does a split (see below) using the <prefix> supplied and then does a git push to push the result to the repository and ref. This can be used to push your subtree to different branches of the remote repository.

split

Extract a new, synthetic project history from the history of the <prefix> subtree. The new history includes only the commits (including merges) that affected <prefix>, and each of those commits now has the contents of <prefix> at the root of the project instead of in a subdirectory. Thus, the newly created history is suitable for export as a separate git repository.

After splitting successfully, a single commit id is
printed to stdout.  This corresponds to the HEAD of the
newly created tree, which you can manipulate however you
want.

Repeated splits of exactly the same history are
guaranteed to be identical (ie. to produce the same
commit ids).  Because of this, if you add new commits
and then re-split, the new commits will be attached as
commits on top of the history you generated last time,
so ‘git merge’ and friends will work as expected.

Note that if you use ‘–squash’ when you merge, you
should usually not just ‘–rejoin’ when you split.

 

OPTIONS

-q, –quiet

Suppress unnecessary output messages on stderr.

-d, –debug

Produce even more unnecessary output messages on stderr.

-P <prefix>, –prefix=<prefix>

Specify the path in the repository to the subtree you want to manipulate. This option is mandatory for all commands.

-m <message>, –message=<message>

This option is only valid for add, merge and pull (unsure). Specify <message> as the commit message for the merge commit.

 

OPTIONS FOR ADD, MERGE, PUSH, PULL

–squash

This option is only valid for add, merge, push and pull commands.

Instead of merging the entire history from the subtree
project, produce only a single commit that contains all
the differences you want to merge, and then merge that
new commit into your project.

Using this option helps to reduce log clutter. People
rarely want to see every change that happened between
v1.0 and v1.1 of the library they’re using, since none of the
interim versions were ever included in their application.

Using ‘–squash’ also helps avoid problems when the same
subproject is included multiple times in the same
project, or is removed and then re-added.  In such a
case, it doesn’t make sense to combine the histories
anyway, since it’s unclear which part of the history
belongs to which subtree.

Furthermore, with ‘–squash’, you can switch back and
forth between different versions of a subtree, rather
than strictly forward.  ‘git subtree merge –squash’
always adjusts the subtree to match the exactly
specified commit, even if getting to that commit would
require undoing some changes that were added earlier.

Whether or not you use ‘–squash’, changes made in your
local repository remain intact and can be later split
and send upstream to the subproject.

 

OPTIONS FOR SPLIT

–annotate=<annotation>

This option is only valid for the split command.

When generating synthetic history, add <annotation> as a
prefix to each commit message.  Since we’re creating new
commits with the same commit message, but possibly
different content, from the original commits, this can help
to differentiate them and avoid confusion.

Whenever you split, you need to use the same
<annotation>, or else you don’t have a guarantee that
the new re-created history will be identical to the old
one.  That will prevent merging from working correctly.
git subtree tries to make it work anyway, particularly
if you use –rejoin, but it may not always be effective.

-b <branch>, –branch=<branch>

This option is only valid for the split command.

After generating the synthetic history, create a new
branch called <branch> that contains the new history.
This is suitable for immediate pushing upstream.
<branch> must not already exist.

–ignore-joins

This option is only valid for the split command.

If you use ‘–rejoin’, git subtree attempts to optimize
its history reconstruction to generate only the new
commits since the last ‘–rejoin’.  ‘–ignore-join’
disables this behaviour, forcing it to regenerate the
entire history.  In a large project, this can take a
long time.

–onto=<onto>

This option is only valid for the split command.

If your subtree was originally imported using something
other than git subtree, its history may not match what
git subtree is expecting.  In that case, you can specify
the commit id <onto> that corresponds to the first
revision of the subproject’s history that was imported
into your project, and git subtree will attempt to build
its history from there.

If you used ‘git subtree add’, you should never need
this option.

–rejoin

This option is only valid for the split command.

After splitting, merge the newly created synthetic
history back into your main project.  That way, future
splits can search only the part of history that has
been added since the most recent –rejoin.

If your split commits end up merged into the upstream
subproject, and then you want to get the latest upstream
version, this will allow git’s merge algorithm to more
intelligently avoid conflicts (since it knows these
synthetic commits are already part of the upstream
repository).

Unfortunately, using this option results in ‘git log’
showing an extra copy of every new commit that was
created (the original, and the synthetic one).

If you do all your merges with ‘–squash’, don’t use
‘–rejoin’ when you split, because you don’t want the
subproject’s history to be part of your project anyway.

 

EXAMPLE 1. ADD COMMAND

Let’s assume that you have a local repository that you would like to add an external vendor library to. In this case we will add the git-subtree repository as a subdirectory of your already existing git-extensions repository in ~/git-extensions/:

$ git subtree add –prefix=git-subtree –squash \
        git://github.com/apenwarr/git-subtree.git master

master needs to be a valid remote ref and can be a different branch name

You can omit the –squash flag, but doing so will increase the number of commits that are included in your local repository.

We now have a ~/git-extensions/git-subtree directory containing code from the master branch of git://github.com/apenwarr/git-subtree.git in our git-extensions repository.  

EXAMPLE 2. EXTRACT A SUBTREE USING COMMIT, MERGE AND PULL

Let’s use the repository for the git source code as an example. First, get your own copy of the git.git repository:

$ git clone git://git.kernel.org/pub/scm/git/git.git test-git
$ cd test-git

gitweb (commit 1130ef3) was merged into git as of commit 0a8f4f0, after which it was no longer maintained separately. But imagine it had been maintained separately, and we wanted to extract git’s changes to gitweb since that time, to share with the upstream. You could do this:

$ git subtree split –prefix=gitweb –annotate='(split) ‘ \
        0a8f4f0^.. –onto=1130ef3 –rejoin \
        –branch gitweb-latest
$ gitk gitweb-latest
$ git push [email protected]:whatever/gitweb.git gitweb-latest:master

(We use 0a8f4f0^.. because that means "all the changes from 0a8f4f0 to the current version, including 0a8f4f0 itself.")

If gitweb had originally been merged using git subtree add (or a previous split had already been done with –rejoin specified) then you can do all your splits without having to remember any weird commit ids:

$ git subtree split –prefix=gitweb –annotate='(split) ‘ –rejoin \
        –branch gitweb-latest2

And you can merge changes back in from the upstream project just as easily:

$ git subtree pull –prefix=gitweb \
        [email protected]:whatever/gitweb.git master

Or, using –squash, you can actually rewind to an earlier version of gitweb:

$ git subtree merge –prefix=gitweb –squash gitweb-latest~10

Then make some changes:

$ date >gitweb/myfile
$ git add gitweb/myfile
$ git commit -m ‘created myfile’

And fast forward again:

$ git subtree merge –prefix=gitweb –squash gitweb-latest

And notice that your change is still intact:

$ ls -l gitweb/myfile

And you can split it out and look at your changes versus the standard gitweb:

git log gitweb-latest..$(git subtree split –prefix=gitweb)

 

EXAMPLE 3. EXTRACT A SUBTREE USING BRANCH

Suppose you have a source directory with many files and subdirectories, and you want to extract the lib directory to its own git project. Here’s a short way to do it:

First, make the new repository wherever you want:

$ <go to the new location>
$ git init –bare

Back in your original directory:

$ git subtree split –prefix=lib –annotate="(split)" -b split

Then push the new branch onto the new empty repository:

$ git push <new-repo> split:master